Mr.Vortex
I have notice that most of the time when jet engine has raised their RPM
, the engine oil temperature will decrease regardless of flight condition.
That is partially true, but only for a very short while. In a high speed bearing application such as that in a jet engine, a significant source of heat generation is that of the energy required to pump the oil from the bearing itself. At very high speeds this is generally the most significant source of heat. In a jet engine, as oil flow is increased, say from ground idle to TO thrust, the bearing temperatures will at first fall to some lower value, but then will steadily increase. They reason is a drag loss caused by the bearing ploughing through oil that is momentarily trapped in the bearing before being flung out by centrifugal force. The term is called "churning loss". The temperature rises because the churning losses overpowers the oil's ability to remove the heat generated. The temperature generally begins to settle to a norm once the climb out phase of the flight is accomplished. Oil from bearing sump returns is pumped through a cooler where it is cooled by main engine fuel flow coming from the aircraft wings.
I should add, at high altitudes during the cruise portion of the flight, oil cooling is switched to the air oil cooler for those engines that have both oil cooling systems.
Turbine D